This invention relates to a color filter and a solid state imaging apparatus provided with this color filter.
As is well known, color television systems such as NTSC, PAL and SECAM are used in many countries. The interlaced scanning method is a standard method for detecting and representing the image information in these systems.
In order to reproduce an image on the usual receiver, it is necessary that the solid state imaging apparatus matches with these systems.
FIG. 1 is a diagram for the explanation of interlaced scanning. In this case, a character, e.g., A, is focussed onto the imaging surface of a solid state imaging device from which an image signal is derived. This figure shows a state in which photoelectric conversion is done at each scanning from top to bottom and from left to right, whereby a video signal corresponding to the image is detected. First, interlaced scanning is done in the order of 1A, 2A, 3A, . . . MA. After the scanning of the A field, scanning of the B field is done in the order of 1B, 2B, 3B, . . . , MB so that intermediate regions of the image between scanning lines 1A and 2A, 2A and 3A etc. are filled. By the scannings of the A-field and the B-field, a complete image of A, i.e. the whole image surface (1 frame), is constituted. In the NTSC system using the interlaced scanning, a television image is composed of 30 frames per second; 30 scannings per second of the A-field and 30 scannings per second of the B-field. According to this standard scanning method, video signals are read from each photoelectric element every 1/30 second. Such a method of reading out the signal is called the "frame integration mode".
On the other hand, the following method is used in the field of solid state imaging apparatus. According to this method, simultaneous scannings of line pairs (1A, 1B), (2A, 2B), (3A, 3B), . . . are performed first. In the A-field, signals are derived through respective signal lines and mixed in an external circuit as 1A+1B, 2A+2B, . . . . Then, the B-field scannings is performed in such a manner that line pairs (1B, 2A), (2B, 3A), (3B, 4A), . . . with one line shifted downwards are read out and mixed as 1B+2A, 2B+3A, 3B+4A, . . . . Scanning of one frame is thus finished. In this case, video signals are read from each photoelectric elements every 1/60 seconds.
This method is called the "field integration mode". Especially, the solid state imaging device has substantial merit in comparison with image tubes in that, in view of its principle of operation, it is free from the problem of a burned-in, and additionally has a low degree of residual image. In order to make the best use of this characteristic, interlaced scanning with the field integration mode is often desired, although the vertical resolution is more or less sacrified.
In order to constitute a single chip type color camera with one solid state imaging device, each picture element in the solid state imaging device 11 should be adjusted and mounted with each element of a color filter 12, as shown in FIG. 2. That is, picture elements A.sub.11, A.sub.12, . . . , A.sub.21, A.sub.22, . . . , etc. should be adjusted respectively and accurately with color filter elements C.sub.11, C.sub.12, . . . , C.sub.21, C.sub.22, . . . , etc.
External light 13, after passing through each color filter element C.sub.11, C.sub.12, . . . , is focused on each picture element A.sub.11, A.sub.12, . . . , and converted into signal charge. In the case of reading out the A-field where lines 21 and 22 are scanned, signal charges for pair picture elements A.sub.11 and A.sub.21, A.sub.12 and A.sub.22, . . . , are mixed. When lines 23 and 24 are scanned, signal charges for pair picture elements A.sub.31 and A.sub.41, A.sub.32 and A.sub.42, A.sub.33 and A.sub.43, . . . are mixed. When lines 25 and 26 are scanned, signal charges for pair picture elements A.sub.51 and A.sub.61, A.sub.52 and A.sub.26, A.sub.53 and A.sub.63, . . . are mixed.
Next, in the case of reading out the B-field where lines 22 and 23 are scanned, signal charges for pair picture elements A.sub.21 and A.sub.31, A.sub.22 and A.sub.32, A.sub.23 and A.sub.33, . . . are mixed.
FIG. 3 shows the constitution unit of a color filter using the field integration mode. The unit consists of transparent filter elements (W) C.sub.11, C.sub.31, C.sub.51 ; green light transmission filter elements (G) C.sub.12, C.sub.32, C.sub.52 ; yellow light transmission filter elements (Ye) C.sub.21, C.sub.42 ; and cyan color transmission filter elements (Cy) C.sub.22, C.sub.41. The numerals 21 to 25 denote lines. Transparent filter elements transmit elementary color components red, blue and green; yellow light transmission filter elements transmit red and green primary light components; and cyan color transmission filter elements transmit blue and green primary color components.
In this color filter, transparent filter elements having a high light transmission factor are used. Therefore, it is a merit that the sensitivity becomes high in the region of these elements. However, since the transmission factors of Cy, Ye, W and G color filter elements are different, the amounts of signal charge from each picture element are different. The G-filter element has the minimum utilization factor of light. Hence, the amount of light necessary for saturating a picture element which is constituted by a photoelectric conversion element corresponding to a G-filter element becomes about three times as large as that necessary for a W-filter element. Even if other picture elements are not yet saturated, it happens that picture elements corresponding to W-filter elements are saturated. As a result, the dynamic range of the elements is limited by the region of W-filter elements. Consequently, and in contrast with a filter where the intensities of light transmitting through all the filter elements are nearly equal, the dynamic ranges of the picture elements, particularly of the picture elements corresponding to G-filter elements, are not fully utilized and thereby decreased.